In general, a three-way catalyst is used in order to detoxify exhaust gas which is discharged from an internal combustion engine such as an automotive engine and contains hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx). As an example of the three-way catalyst, one in which noble metal particles of platinum (Pt), palladium (Pd), rhodium (Rh) and the like are carried on a carrier of alumina (Al2O3) or the like as a porous body is given.
As another example of the three-way catalyst, a three-way catalyst in which one formed by carrying ones to be described below on active alumina by an impregnation method are carried on a honeycomb substrate is proposed. The ones carried on the honeycomb substrate are: at least one selected from cerium (Ce), zirconium (Zr), iron (Fe) and nickel (Ni); at least one selected from Pt, Pd and Rh; and at least one selected from neodymium (Nd), lanthanum (La) and praseodymium (Pr) according to needs (refer to Japanese Patent Laid-Open Publication No. S59-230639 (published in 1984)).
A purification reaction for the exhaust gas is a catalytic reaction progressing on a surface of the noble metal. Accordingly, a catalytic activity of the noble metal is substantially proportional to a surface area of the noble metal. Therefore, in order to obtain the maximum catalytic activity from a small amount of the noble metal, it is preferable to produce noble metal particles with a small particle diameter and a large specific surface area, and to evenly disperse the noble metal particles on the porous carrier while maintaining the particle diameter.